Expression of heterologous DNA sequences in a plant host is dependent upon the presence of an operably linked promoter that is functional within the plant host. The type of promoter sequence chosen is based on when and where within the organism expression of the heterologous DNA is desired. Where expression in specific tissues or organs is desired, tissue-preferred promoters may be used. Where gene expression in response to a stimulus is desired, inducible promoters are the regulatory element of choice. In contrast, where continuous expression is desired throughout the cells of a plant, constitutive promoters are utilized. Additional regulatory sequences upstream and/or downstream from a core promoter sequence may be included in expression constructs of transformation vectors to bring about varying levels of expression of heterologous nucleotide sequences in a transgenic plant. Genetically altering plants through the use of genetic engineering techniques to produce plants with useful traits thus requires the availability of a variety of promoters.
Frequently it is desirable to express a DNA sequence in particular tissues or organs of a plant. For example, increased resistance of a plant to infection by soil- and/or air-borne pathogens might be accomplished by genetic manipulation of the plant's genome to comprise a tissue-preferred promoter operably linked to a heterologous pathogen-resistance gene such that pathogen-resistance proteins are produced in the desired plant tissue.
Alternatively, it might be desirable to inhibit expression of a native DNA sequence within a plant's tissues to achieve a desired phenotype. In this case, such inhibition might be accomplished with transformation of the plant to comprise a tissue-preferred promoter operably linked to an antisense nucleotide sequence, such that expression of the antisense sequence produces an RNA transcript that interferes with translation of the mRNA of the native DNA sequence.
To date, the regulation of gene expression in plant roots has not been adequately studied despite the root's importance to plant development. To some degree this is attributable to a lack of readily available, root-specific biochemical functions whose genes may be cloned, studied, and manipulated. Several genes that are preferentially expressed in plant root tissues have been identified. See, for example, Takahashi et al. (1991) Plant J. 1:327-332; Takahashi et al. (1990) Proc. Natl. Acad. Sci. USA 87:8013-8016; Hertig et al. (1991) Plant Mol Biol. 16:171-174; Xu et al. (1995) Plant Mol. Biol. 27:237-248; Capone et al. (1994) Plant Mol. Biol. 25:681-691; Masuda et al. (1999) Plant Cell Physiol. 40(11):1177-81; Luschnig et al. (1998) Genes Dev. 12(14):2175-87; Goddemeier et al. (1998) Plant Mol. Biol. 36(5):799-802; and Yamamoto et al. (1991) Plant Cell 3(4):371-82. Though root-specific promoters have been characterized in several types of plants, no root-specific promoters from maize have been described in the literature.
Constitutive expression of some heterologous proteins, such as insecticides, leads to undesirable phenotypic and agronomic effects. Limiting expression of insecticidal proteins, for example, to the target tissues of insect feeding (root, in this case), allows the plant to devote more energy to normal growth rather than toward expression of the protein throughout the plant. Using root-preferred promoters, one can also limit expression of the protein in undesirable portions of the plant. However, many of the root-preferred promoters that have been isolated do not direct the expression of sufficient amounts of a transgene for efficacy in plants. Thus, the isolation and characterization of tissue-preferred, particularly root-preferred, promoters that can direct transcription of a sufficiently high level of a desired heterologous nucleotide sequence is needed.